This invention relates to the continuous casting of high strength, light metal alloys and to the continuous casting of lithium-containing alloys such as aluminum-lithium alloys.
The process of continuously casting high strength, light metal alloys into acceptable ingots of large size depends on the manner of cooling. Large size ingots include ingots having a cross section larger than about six inches in thickness (e.g., rectangular ingot for rolling mill stock) or larger than about six inches in diameter (e.g., round ingot for forgings or extrusions). Cooling method and rate influence the ingot's tendency to form undesirably brittle or low strength structures, such as edge cracking or surface cracking when the large cross section ingot subsequently is rolled.
Large ingots of high strength light metal are produced conventionally by continuous or semicontinuous direct chill casting using water coolant. A continuous ingot having a solid surface but a core which is still molten is formed in a water-cooled mold. After passing through the mold, water exits directly on the hot solid ingot surface to provide a direct chill cooling. The water then separates or falls from the ingot after extracting heat. Typically, this water is collected in a pool or reservoir in the casting pit.
However, bleed-outs occasionally occur in which molten metal from the ingot core flows through a rupture in the solid wall or shell of the ingot, and liquid metal comes into direct contact with the water. Bleed-outs tend to be more severe with larger size ingots. A Tarset (e.g., a coal tar epoxy) or an equivalent protective coating is applied to steel and concrete surfaces in the casting pit, which surfaces otherwise would be exposed to water and molten metal spilled in the pit. The Tarset provides significant protection from explosion.
Lithium-containing alloys are considered to have substantial promise for high technology applications such as aircraft plate, sheet, forgings, and extrusions. Light metal lithium-containing alloys, such as aluminum-lithium alloys, are highly regarded by reason of material properties such as low density, high strength, high modulus of elasticity, and high fracture toughness. The combination of these material properties can reduce the weight of large commercial airliners by as much as six tons or more. The resulting weight savings can reduce an aircraft's fuel consumption by 220,000 gallons or more during a typical year of operation.
However, a significant processing obstacle stands in the way of the substantial development of large-scale lithium-containing alloy applications such as plate and sheet. This processing problem has prevented the production of a sufficiently large ingot which would permit the formation, e.g., by rolling, of large plates or sheets.